Fax¶
D-Modem can work as a fax modem for fax programs on your computer, and as a complete fax machine of its own for the phone-system modules and libdmodem. Faxes can go over an ordinary phone call's audio or over the internet using T.38.
For every command, frame and timing, see the technical details.
Fax modem for fax programs¶
Fax programs (HylaFAX, efax, Windows Fax and Scan and so on) talk to a fax modem with one of three command sets. D-Modem has all of them:
| Command set | Turn on with | Who does the fax protocol |
|---|---|---|
| Class 1 | AT+FCLASS=1 |
the fax program; the modem only sends and receives the signals |
| Class 1.0 | AT+FCLASS=1.0 |
as class 1, with V.34 fax too |
| Class 2 | AT+FCLASS=2 |
the modem |
| Class 2.0 | AT+FCLASS=2.0 |
the modem (the standardised version of class 2) |
Class 1 is the most widely supported and the most flexible. Classes 2 and 2.0 suit simpler programs.
The fax speeds are the usual ones: V.27ter (2400 and 4800 bit/s), V.29
(7200 and 9600) and V.17 (up to 14400). In class 2 and 2.0, V.34 fax
goes up to 33600 bit/s when the other machine has it too: set the
highest speed to 13 with error correction on (for example
AT+FIS=1,13,0,2,0,2,0,0). A machine without V.34 still gets the
usual speeds. To poll (fetch a document) on V.34, send AT+FSP=1
before dialling.
In class 1.0 (AT+FCLASS=1.0), fax programs that know V.34 fax, such as
HylaFAX, get it the same way: the modem says +F34: with the speed
before its CONNECT, and the program sends and receives the pages at
up to 33600 bit/s. AT+F34= sets the highest and lowest speed (in
steps of 2400 bit/s; AT+F34=0 turns V.34 off). With a machine that has
no V.34, the call carries on as ordinary class 1.
Built-in fax machine¶
D-Modem also has its own complete fax machine (the T.30 protocol), used by the Asterisk, FreeSWITCH and dmodem-ari fax applications and by libdmodem. It sends and receives TIFF fax files, and uses error correction mode when the other machine supports it, so pages arrive without damaged lines.
With error correction on, the pages also go in T.85 (JBIG) coding when the other machine takes it: a better compression than the usual MH coding, so a page of text takes about half the time on the line. Pages that come in T.85 are saved as ordinary fax TIFF files.
It also sends and receives colour and grey-scale pages (T.42, JPEG coding), again with error correction on. A colour or grey page in the TIFF file to send (8-bit RGB or grey, uncompressed) goes in colour when the other machine takes it, in grey when it takes only grey, and as a black-and-white dithered page otherwise. A colour page that comes in is saved in colour in the received TIFF file; programs that take only black and white (class 2 fax software) get a dithered copy.
Pages with few colours, such as charts, diagrams and coloured text, go without any loss (T.43, JBIG coding of the colours) when the other machine takes that: the colours come out as they went in, and the page is far smaller than in JPEG (a bar chart of five colours: 199 octets instead of 25809). Photographs and other pages of many colours still go in JPEG. Pages that come in T.43 are saved like JPEG ones.
Pages with text over a picture go in T.44 (mixed raster content) when the other machine takes it: the text's shapes are kept sharp in black and white, and the picture goes in JPEG at half the resolution underneath. A page of text over a photo is far smaller than in JPEG (the test's page goes in about a quarter of the time) and its text comes out crisper. Pages that come in T.44 are saved like JPEG ones.
Binary files¶
With error correction on, the fax machine can also send and receive
binary files (T.434) in the same call as the pages: any file,
with its name and type, arrives exactly as it was sent. A program
using libdmodem turns this on with t30_config's bft; a machine that
doesn't take files still gets the pages, and the files are left out.
Fax over the internet (T.38)¶
Fax tones do not survive many internet phone calls well. T.38 sends the fax as data packets instead of sound. Both the built-in fax machine and the class 1 fax modem can use T.38, and a call can switch to it in the middle.
Testing¶
The fax tests send pages through each fax speed on a simulated line, run two fax machines against each other (on audio and over T.38, with lost packets), and check D-Modem against spandsp's fax code and against other companies' fax modems. See Building and testing.